1. Field of the Invention
The present invention relates to a radiation detector for use with radiation apparatus such as X-ray computerized tomography apparatus.
2. Description of the Related Art
A radiation detector is often used with X-ray computerized tomography apparatus (hereinafter referred to as X-ray CT apparatus).
The X-ray CT apparatus is a clinically useful apparatus which performs image reconstruction processing, such as successive approximation or Fourier transformation, on projection data obtained from various directions for a predetermined plane of a subject under examination, computes a CT value at each of points representing the plane, and imparts a proper gradation to the CT value, thereby obtaining a tomography image of the plane of the subject. The third-generation X-ray CT apparatus, in which an X-ray tube and a radiation detector turn around a subject to be examined, is constructed as follows.
An X-ray tube emitting X-rays from its focus and a multichannel radiation detector are supported so that they can turn while facing each other with an aperture therebetween. Data acquisition is performed by repeating emission of X-rays and detection of transmitted X-rays each time the X-ray tube and the radiation detector turn around a subject to be examined placed within the aperture through a very small angle.
In the multichannel radiation detector for use with such X-ray CT apparatus, blocks each having an array of many detector elements thereon are arranged in the form of a circular arc with center at the focus of the X-ray tube. On the X-ray tube side of the blocks is disposed a collimator whose collimator plates stand along the direction of emission of X-rays from the focus of the X-ray tube. Each of the collimator plates is placed over the gap between adjacent detector elements.
On the block substrate photodiodes are first, placed and scintillators are next placed on the photodiodes. Also, a housing for supporting the collimator plates is mounted on the block substrate.
FIG. 1 is a sectional view of a radiation detector. In order to shut out scintillations from other detector elements (channels), a reflector 14 (indicated by oblique lines) is interposed between scintillators 12.
FIG. 2 is an enlarged view of a portion indicated by dotted circle in FIG. 1. The thickness T1 of the reflector 14 is determined according to only the pitch of photodiodes 11. Thus, the thickness T1 of the reflector 14 is generally less than the thickness T2 of a collimator plate 8.
The focus of the X-ray tube moves because of thermal expansion of its parts which occurs during operation. In particular, the component of movement along the direction in which the detector elements are arranged (hereinafter referred to as the channel direction) is mainly due to the thermal expansion of cathode parts. The movement of the focus varies the sensitivity of the detector elements. In the following the "movement" refers to the "movement along the channel direction", unless otherwise specified. The reason will be described below with reference to FIG. 3. In FIG. 3, focus 1 is assumed to be the focus maintained at low temperatures, while focus 1' is assumed to be the focus when the quantity of heat stored in the anode plate and parts of the X-ray tube has reached the heat capacity, that is, when focus 1 has traveled the maximum distance. Note that the collimator plates 8 structuring the collimator are aligned with respect to the focus 1 located at low temperatures (at the time of design).
When the focus 1 moves toward the focus 1' as the anode plate expands, the collimator plate 8 makes shade (indicated by oblique lines) on the detecting surface of the scintillator 12. The shade reaches its maximum when the focus of the X-ray tube is located to the point 1', resulting in a decrease in the original width W1 of the detecting surface of the scintillator 12 by W2. That is, the channel sensitivity decreases gradually as shown in FIG. 4 as the focus moves. The variations in channel sensitivity can be compensated for easily.
However, the variations in channel sensitivity differ from channel to channel. This is due to the fact that the focus 1 does not move on a circular arc to fit the arrangement of the collimator plates and thus the degree of growth of the shade varies from channel to channel. The difference in sensitivity between channels causes a problem that a ring-like artifact may appear in a tomography image.
The compensation for lack of uniformity of sensitivity among channels has been made so far. However, this compensation is limited to compensation for differences in the fluorescent characteristic of scintillators and in the photoelectric transfer characteristic of photodiodes, and no attention is paid to the differences in the degree of growth of the shade of the collimator plates among channels. The compensation for the difference in sensitivity between channels due to the difference in the degree of growth of the shade of the collimator plates between channels requires a lot of work of measuring the variation in sensitivity caused by the movement of the focus of the X-ray tube and obtaining a compensation curve for each channel. This is virtually impossible.
The conventional radiation detector also has the following problem. This arises from collimator alignment errors. That is, each of the collimator plates is not always located accurately between detector elements nor formed accurately along the direction of emission of X-rays from the X-ray tube. Depending of the severity of the alignment errors, the size of the shadow of the collimator plate on the detecting surface of the scintillator may differ from channel to channel. Thus, the alignment errors further increase the difference in sensitivity between channels.